Minerals to be extracted are generally removed from the exposed face of a deposit. The face may, for example, be the wall or the end of a tunnel, stope room, or bore. When the generation of gases is not objectionable, it is common practice to drill holes in a working face, place dynamite in it, blast the face to rubble, and remove the rubble. Frequently, and especially in very deep mines, the generation of gases from blasting is intolerable, and percussive impact techniques are used instead.
When percussive techniques are used, a tool bit is struck by a hammer mechanism, and is thereby driven against and into the face to fracture and dislodge material from it. These techniques are widely used, and there are many tool bits for the purpose.
Existing tool bits suffer from a number of inherent disadvantages, and themselves raise problems for the systems in which they are used. For example, the working room available in many important mines is often very limited. As an example, in gold mines where a vein of rather small thickness is to be extracted, the headroom may be very low. Tools used in such environments are often mounted so that they move along an arc, while their support vehicle crawls along a directed path. In the course of this scoop-like movement, the tool bit is driven against the face to fracture it. A percussive hammer delivers successive blows to the base of the bit. These hammers have substantial axis and lateral dimensions, and the tool bit itself adds significantly to the axial length of the assembled tool and bit.
Material on the face is most advantageously removed by a chipping action in which a blow is delivered at an acute angle to the face. There are considerable potential advantages in making this angle as small as possible, but existing tools render this difficult to the point of practical impossibility. Consider the use of a spike-like single point bit on the central axis of the tool. If the bit is relatively short, then the tool body strikes the mine face within a relatively small angle from the normal to the face. The "cure" for this problem is to elongate the bit, but then the assembly of tool and bit becomes so long as to be unwieldy in many mining situations, and the bits must be made very heavy. Furthermore, the blows are delivered at a single point, which action does not optimally dislodge the face material and is subject to rapid wear. For all of its faults, this arrangement is widely used, because on balance it has the fewest drawbacks in the state of the art as it exists prior to this invention.
One could surmise that a spade-like bit could overcome at least the disadvantage of such a localized exertion of force. However, to resist the resulting substantial banding loads, this type of bit must also be made quite heavy. Worse still, because the rock face is irregular, one edge can dig in before the other, and as the tool is driven, a twisting torque is exerted which either bends or breaks the tool bit, or if the bit survives, the twisting force is exerted through to the tool to the extent that it can actually up-end the tool assembly and turn it over. Spade-like tools are not suitable.
In addition to the foregoing problems, it is inherent in conventional digging type tools that at the very point of impact where they are most heavily loaded, coolant gases or liquids cannot reach into the pocket created by the tool. Thus, at the point of maximum stress, the tool is subjected to the highest temperatures, and this leads to greatly increased tool wear.
A tool bit according to this invention is not only axially shorter than a conventional center-spike tool must be, but can be made so short as to enable the angle of attack to be importantly smaller than was before attainable. It also enables a scoop-like blow to be given without exertion of a twisting torque on the system. As a consequence, scoop-type impact can be delivered by a machine with importantly reduced axial length and reduced interference.
Because this tool is rotated during usage, it continually presents to the working face a fresh and cooled cutting edge. Each length of cutting edge is steadily replaced, so there is no significant temperature build-up. The tool life is greatly extended. Further, a circular configuration is much more resistant to bending forces than a spade.
Examples of mining operations in which this invention provides important advantages are in the extraction of quartzite gold ores, and in coal deposits wherein the coal itself is so close to layers of shale that a mixed product results which would tend to destroy conventional toothed mining machinery.